Due to their robustness, capacitive fill level measurements are used in many technical fields.
Here, the dominant principle is that a conductive rod or a conductive rope or cable functioning as an electrode is arranged insulated in a conductive container. The container wall and the electrode form a capacitor arrangement. The electric field of this capacitor arrangement thus fills the entire container volume. If this container volume is filled with a dielectrically acting material, the capacitance value of this capacitor arrangement is changed.
Since all technical products have a higher dielectric action with respect to air, which is designated as permittivity, the capacitance value is continuously increased when filling the container, in particular in the case of a positionally stable, substantially vertical mounting in the container and a suitable embodiment of the rods or cables or ropes, which are designated as probes or measuring sensors, and which electrically represent an electrode of the capacitor arrangement with respect to the container wall as the second electrode. The capacitance change between the empty state and the different fill states is detected by suitable electronic measuring electronics and converted into the desired analogue or switching output signals.
An electronic evaluation of the electrical primary signals obtained by means of capacitive measuring sensors is known from DE 1275777, for example.
Although usually fill level measurements are spoken of, at least one indication about the volume or the mass of the material in the container is desired. If there is a linear correlation between the fill height and the volume increase, this indication can be readily provided since only one constant factor is to be taken into account.
For this, the following requirements are to be met: The cross-section of the container remains constant over the height and the fill level measurement delivers reproducible results. As already described, the signal of the fill level measurement is determined through a comparison of the corresponding capacitance value of the respective current fill level with the capacitance value of the empty state.
However, many materials cause adhesions both on the container and the measuring sensors due to electrostatics or high viscosities, for example. Since adhesiveness is influenced by material change, different process parameters such as temperature, humidity and/or grain size, the comparative value of the “empty” container often can no longer be considered as being constant. In the case of unfavorable container geometries, this can result in uselessness of the measurement.
This problem can partially be alleviated by detecting the comparative value “empty container”, that is to be determined through calibration in a real situation, thus by using typical material approaches. However, with the dependencies mentioned above, this requires frequent corrections of the calibration during changing conditions.
The container geometry that is ideal for a meaningful fill level measurement effects a linear capacitance increase over the fill height. For practice, this is always provided to a sufficient extent if at constant container cross-section, the ratio between height and diameter of the container is high. A cylindrical capacitor in the interior of which the electrical field provides these requirements in an ideal manner can be viewed as an example for this. However, at the ends of the cylindrical capacitor there are “boundary distortions” of the electrical fields which result in more or less significant nonlinearities.
For many container shapes used in practice, a quantitative assessment about the influence of boundary distortions on the linearity of the fill level measurements is hardly possible. Specifically in the case of the frequent request to implement the fill level range at a maximum from the bottom up to the upper edge, no theoretical prediction is possible and much less, a calculation of the linearity error is possible.
Besides the previously described principle of a capacitor formed from a conductive container wall and a probe that is insulated with respect thereto, furthermore, another version is known in which 2 electrodes are arranged in the container at a constant distance from one another and insulated with respect to one another. For instance, for measurements in low-viscosity liquids in the container interior or as a bypass, a slim tube (measuring tube) with a centrically arranged inner electrode is used as a measuring arrangement. This arrangement corresponds to the above-described cylindrical capacitor that has a high ratio between height and diameter. Changes in the cross-section of the container thus have no influence on the measuring signal.
However, this principle cannot be used for high-viscosity liquids and bulk materials since no identical fill level in the container and the measuring tube can be guaranteed.
This problem can also be solved with another variant in the form of electrodes that are arranged in parallel. Here, two electrodes in the form of wires or strips that are insulated from one another are used. However, since in this case the electrical field concentrates primarily between these parallel electrodes, and the material to be detected is detected only to a small extent by this measuring field, such an arrangement leads to unsatisfactory results.
This situation can be improved by providing shielding between the electrodes so that a measuring field can form in the outer region of the electrodes arranged in parallel.
Numerous variants for specific configurations of such a shielding are known, both for mechanical implementation in the measuring sensors and for obtaining a suitable shielding potential within the measuring electronics.
In terms of their measuring behavior, the measuring arrangements described last thus are largely independent of container geometries if a minimum distance is maintained between a measuring sensor and the container wall. This distance should be dimensioned such that the measuring field at the wall comprises only a negligible portion of the total field. This requirement corresponds to the natural aim of establishing a sufficient distance between the wall and the sensor so as to prevent material bridging. The disadvantage of measuring sensors having parallel electrodes is however the strong dependency on material adhesions on the sensor surface since the field portion of the measuring field is greatest directly on the surface.
The few above-mentioned dependencies already show that the requirements for implementing a specific fill level measurement are very complex. In practice, a measurement arrangement is in most cases selected by the user; however, the interrelations of primary signal recovery by the electrical measuring field are not taken into account or are insufficiently taken into account. This, specifically if no professional advice is provided, often results in malfunctions which are attributed to the measuring electronics, but in fact are caused by the wrong selection of the measuring arrangement.
If a malfunction is detected when starting a capacitive measurement, the entire measuring arrangement of the current prior art has to be replaced. In most cases, this involves high expenditures in terms of time and costs.